Capacitor with composite leads of different metals



Feb. 9, 1965 M. M. LAYToN ETAI.

CAPACITOR WITH COMPOSITE LEADS OF DIFFERENT METALS Filed May 5, 1959 @la2ol msm. mm3; man@ f wz?? o@ /.J o wf n me@ A maar Mo; @n.6

United States Patent Orifice 3,169,216 Patentedv Feb. 9, 1965 Thisinvention relatesgenerally to electrical capacitors of the typedescribed in U.S. Patent No. 2,696,577, issued to G. P. Smith. It isparticularly concernedi with improvements in capacitor encapsulation andelectrical lead design to provide a hermetically encapsulated capacitor.

In the glass capacitor of the Smith patent, thin strips of glass areemployed as dielectric material and are interleaved with metal foilelectrodesto form a stack. The stack is encased between shaped glasscovers,-and the entire assemblyconsolidated by application of heat andpressure to produce a glass encapsulated glass capacitor.'

' Such capacitors provide superior electrical properties and a compactintegral construction well adapted to miniaturized electrical systems.Experience has demonstrated, however, that utilization under certainenvironmental conditions requires a combination of properties which hasheretofore appeared incompatible. Among these properties are relativelyflexible electrical leads, a high Q value in addition to otherelectrical characteristics, and means for excluding moisture from theencapsulated capacitor.

Flexible electrical leads facilitate assembly of capacitors intoelectrical systems and minimize transmission of stress to the fragilecapacitor element and encapsulation. However, the conventional flexiblewire materials, brass and copper, have thermal coeiiicients of expansionmuch higher, than ordinary glasses. Adhesion of thecover glass to such awire lead during the consolidationstep cause stresses to developoncooling. These stresses resultin glass fracture, and it has,therefore, been considered necare avoided and the described requirementsare fulfilled. A further purpose is to provide a hermeticallyencapsulated capacitor. Another purpose is to provide an improvedelectrical lead for encapsulated capacitors. A

,more specific purpose is to provide a glass encapsulated capacitorhaving a strong, crack-resistant construction and a fusion seal betweenthe electrical leads of the capacitor and the glass encapsulation.

The present invention provides a hermetically encapsulated capacitorcomprising a capacitor element composed of electrodes separated bydielectric material, electrically conducting leads attached to theelectrodes and extending outwardly through a glass casing thatencapsulates'the element, each such lead being composed of a relativelyinflexible portion, such portion being attached n to an electrode andfusion sealed into the glass casing and having an effective thermalcoefficient of expansion matching that of the glass, and aiiexiblesecond portion which is integrally attached to said firstportion but wholly external of the glass casing. `The invention ishereafter described with specic reference to a glass capacitor and inconjunction with the accompanying drawing in which:

FIG. l is an exploded sectional view of a glass capacitor assembly, n

FIG.; 2 is an enlarged, partly in section, view of an electrical lead ofFIG. l, FIG. 3 is an elevational view in section showing the assembly ofFIG. l in consolidated form,

FIG. 4 is a fragmentary elevational view in section of the capacitorassembly of FIG. V3 and illustrating the essary to avoid such adhesionin producing glass encased capacitors in accordance with, the Smithpatent'.

Under conditions of high humidity, salt water spray and the like,moisture entersthe capacitor assembly along the lead wire. This has beenVfound to promote an electrochemical 4reaction at the terminal junctionwhich results y in failure of .thecapaciton Organic sealants yhavefailed to provide acomplete barrier to such moisture entry.

Amongthe various electrical characteristics ofcapacitors, prescribed`interms of minimal limits by specificationsffsuchasmilitaryspecitication MIL-C-11272A, is

the 'capacitor Q value.H As is.well known, this valueis a dimensionlessigureof ,meritfor capacitors that varies inversely with .theresistanceimparted by various capacitor assembly components., It isnumerically equaltothe ratio of reactanceto resistance'and may also beexpressed as the reciprocalof the dissipation factor.

In glass capacitors,'optimum Q values are obtained by producing theglass .dielectric film and the encapsulating covers from ay glasscfthetype disclosed in U.S. Patentv rNo. 2,527,693,` issued toW. H.Arrnistead. Theseglasses j;

formation of a fusion seal in accordance with the present invention, andi FIG. Sis a side View, partly in section, of a capacitor encapsulatedin accordance with the invention.

The capacitor assembly of FIG.,1k includes Va capacitor elementplnz,opposed glass covers 14-14 and composite electrical leads lr6-4.6,'Except Afor leads 16, the components ofthe capacitor may be formed andassembled in `accordance. with procedures set forth in the earliermentioned Smith patent and prior patents there referred Details,notrepeated here, are incorporated from such patents by reference.'

Capacitor element l12 is composed yof-thin layers of glass 18interleaved with'metal lfoils 20. The latter are alternately offset Vtoprovideterminal portions 22 extending beyond the glass.K Additionallayers of metail foil and glass may be provided if desired for'additional capacitancef' Glass cver's 14- are arranged in opposedrelationship toenclose capacitor element 12, with shaped lateral edgesoverlying the leads as taught by the Smith patent.

- The assembly is shown inexplodedv form, and dimensions are somewhatexaggeratedfand l`is'tortedf,l in' oder to better portray thearran'gedcomponents. f i' Compositeleady members 16 constitute a key feature ofthis invention' and, as shown inexaggerated form-k in FIG. 2,arecompfosed of two distinct/but integral, portions, flag 'member 24 andwirel member' 26. Flag 24 is a'srniall rectangular piece of metal ontheorder of l20-35 mils thick and may, for example, have Vs inch -byijinch sides, the actual size being-suitably proportioned to Yothercapacitor components. Itr composed of a base material 28, which may be.a suitable ferrous 'alloy 'such as 430 Ti stainless steel or aniron-nickel alloy, and surface layers 30 having a thickness of 1-2 mils,the surface layer being composed of :a high conductivity metal such ascopper applied by cladding or plating methods.

The flags are conveniently cut from la large strip or sheet of copperclad metal, thus providing unclad side sections.Y A base metal or alloyis selected to provide thermal expansion characteristics compatible withglass materials to which it issubsequently sealed. The copper clad hasno -,apparent effect in this respect presumably because there are uncladsurfaces on the ag member. A 52% nickel- 48% iron alloy is preferred foruse with glasses having an expansion of 1GO-105 X10-'1` cm./cm./ C.since it provides an expansion match and is easily clad with copper.

The clad or plated surface layer is required to provide a lowresistivity path for electrical current iiow between electrodes 22 andwire member 26. Without the clad layer, capacitor Q values are renderedprohibitively low by the higher resistivity of the base alloy requiredfor sealing purposes. The wire portion 26 of lead 16 may be of anyconventional flexible material, such as brass, copper or silver, that iscapable of being welded or otherwise integrally united to ag 24. Wiremember 26 of lead 16 may be butt welded to flag portion 24 on an uncladsurface. In assembling a capacitor, lead 16 is arranged with the copperclad surfaces 30 of flag portion 24 facing the overlying edge portionsof glass covers 14 and with one such clad surface electricallyconnected, as by welding, to electrode terminal 22.

The components shown in FIG. l are assembled in accordance with knownglass capacitor practice and consolidated into an integral unit bypassing the assembly through pressure rolls while heated to atemperature corresponding to the softening point of the glass which maybe on the order of 500"k C. At this temperature, the glass componentssoften sufficiently to adhere to each other and to` the metal foilsunder the inuence of pressure. However, no appreciable iiow of glassoccurs during this operation as may be seen from FIG. 3 where glasscovers 14 are shown adhered to each other, but with their rounded edges32 substantially unchanged. At higher temperatures, where adequate glassflow for complete sealing would occur, the componentlayers formingeapacitor element 12 would be distorted and damaged. Accordingly, thecapacitor will not be a hermetically sealed unit at this stage.

The fused hermetic seal which characterizes the present invention isprovided in a separate sealing operation illustr-atively shown in FIG.4. As shown in FIG. 3, there is in the consolidated and encapsulatedcapacitor a groove intermediate the rounded edges of glass covers 14. Inhermetically sealing the capacitor, this groove is lled with a softsealing glass 36 which has a suiciently low viscosity lat temperatures`on the order of 450 C. or less to form a fused seal. Sealing glass 36may be a well known soft sealing glass, such asa lead b-orate glass,whose expa-nsion will match with the expansion of the capacitor `glassand Hag Z4. The sealing glass may be iiowed in molten form into thegroove. More conveniently, however, it is powdered and applied in thegroove in a sus- Y pension which is then dried and fused asschematically shown in FIG. 4.

The fragmentary showing of FIG. 4 corresponds to the upper portion ofthe capacitor of FIG. 3 with sealing glass 36 4fused to fill in thegroove and provide a smooth cornposite glass surface. As' schematicallyshown, heat is applied locally, as by heating units 38 to fuse, asealing glass 36 and cause it yto flow and ll the groove.V The amount of1sealing glass employed and the arrangement of lead 16 should be suchthat the sealing glass does not contact wire portion 26 of the compositelead member. Flag portion 24 will be imbedded in sealing glass 36, butmay lextend slightly above the sealing glass surface. It is the functionof sealing glass 36 to completely fillthe groove 4, or valley betweenglass covers 14 and thereby form a fused hermetic seal between flagportion 24 of lead member 16 and the glass cover members.

In this manner the desired hermetic encapsulation of a capacitor isprovided with `a fused seal between the electrically conducting leadmembers and the glass encapsula tion. The inflexible portion of the leadmember provides the necessary expansion match for sealing to the glassencapsulation, while being embedded within the glass Where tiexibilityis immaterial. The flexible portion is out of contact with the sealingglass but providesthe desired exibility so that stresses are nottransmitted into the glass encapsulation of the capacitor. Furthermorethe Q value of the capacitor is maintained at the desired high valuesattained with prior unsealed constructions.

While the invention has been described with reference to a specificglass capacitor element, it will be understood that variousmodifications are possible within the scope of the invention. Inparticular, ceramic materials other than glass may be employed asdielectric in the capacitor element, and the element may alternativelybe assembled by known metallizing procedures.

What is claimed is:

1. A glass encapsulated capacitor comprising a capacitor elementcomposed of electrodes separated by dielectric material and electricallyconducting leads attached to the electrodes and extending outwardlythrough the glass encapsulation, each lead being a composite membercomposed of two distinct parts, one part being a metal that isessentially rigid, attached to the electrodes and fusion sealed directlyto the encapsulating glass, and the second part being composed of adifferent metal that is flexible and integrally attached to the firstpart external of the glass encapsulation.

2. A capacitor in accordance with claim'l wherein the electrodes arecomposed of metal foils, the dielectric rn-aterial is in the form ofthin glass strips, and the glass casing is integrally united with thecapacitor element.

3. A capacitor in accordance with claim l wherein the rst portion of theelectrical lead is composed of a ferrous alloy having a thin layer of ahigh electrical conductivity metalon at least one of its upper and lowersurfaces.

4. A capacitor in accordance with claim 3 wherein the high conductivitymetal is copper.

5. A capacitor in accordance with claim l wherein the fusion sealed,first portion of the electrical lead is in the form of a rectangularflag of relatively large cross section and the outer, second portion isin the for-rn of a wire.

Y References Cited in the file of this patent UNITED STATES PATENTS'2,413,539' Ballard Dec. 31, 1946 2,502,310 Chapman Mar. 28, 19502,552,653 Stupakoif May 15, 1951 2,585,752 Dorst Feb. 12, 1952 2,696,577Smith Dec. 7, 1954 2,750,657 Herbert June 19, 1956 2,825,040 Dorsey Feb.25, 1958 2,921,113 Clemons c Jan. 12, 1960 FOREIGN PATENTS 482,053 GreatBritain M-ar. 23, 1938 V574,501 Great Britain Ian. 8, 1946 585,474 GreatBritain Feb. 7, 1947 785,625 Great Britain Oct. 30, 1957

1. A GLASS ENCAPSULATED CAPACITOR COMPRISING A CAPACITOR ELEMENT COMPOSED OF ELECTRODES SEPARATED BY DIELECTRIC MATERIAL AND ELECTRICALLY CONDUCTING LEADS ATTACHED TO THE ELECTRODES AND EXTENDING OUTWARDLY THROUGH THE GLASS ENCAPSULATION, EACH LEAD BEING A COMPOSITE MEMBER COMPOSED OF TWO DISTINCT PARTS, ONE PART BEING A METAL THAT IS ESSENTIALLY RIGID, ATTACHED TO THE ELECTRODES AND FUSION SEALED DIRECTLY TO THE ENCAPSULATING GLASS, AND THE SECOND PART BEING COMPOSED OF A DIFFERENT METAL THAT IS FLEXIBLE AND INTEGRALLY ATTACHED TO THE FIRST PART EXTERNAL OF THE GLASS ENCAPSULATION. 